Scaffolds have been used extensively in the area of tissue engineering either to construct a neo-tissue that can be implanted to repair a defect site in the body or as a cell container in bioartificial devices. Scaffolds form a three dimensional matrix that serves as a template for cell proliferation and ultimately tissue formation.
Culturing cells in a scaffold typically involves seeding cells throughout the scaffold and allowing the cells to proliferate in the scaffold for a pre-determined amount of time. A lot of research efforts have been directed at the design, fabrication and choice of materials in developing a scaffold for tissue engineering applications. However, the eventual success of a scaffold will be determined by whether the scaffold is able to support cell viability and by its ability to integrate with the host tissues for implantable scaffolds. Hence, the optimization of cell seeding and culture technologies are equally important determinants in the success of a scaffold system.
A first important aspect with regard to cell seeding methods include the efficiency of the seeding method as to maximize the utilization of donor cells. Autologous cell sources are usually limited in number due to donor site morbidity hence, the ideal seeding method need to enable seeding of scaffolds with relatively low cell number at high seeding efficiency. A second important aspect is the uniform distribution of cells in the scaffold. A spatially uniform cell distribution has implications on the formation of a homogeneous tissue. Both factors influence the mass transfer within scaffolds that has been cited as one of the major limitations of culturing cells in a scaffold (Botchwey, E. A., et al., J. Biomed. Mat. Res., 2003, Vol. 67A(1), P. 357-367).
For seeding cells in scaffolds various methods have been described. So far, static seeding is the most prevalent method of seeding cells into scaffolds. For static seeding a cell suspension is seeded on a scaffold and afterwards incubated for a certain time in the absence of agitation before being exposed to dynamic culture conditions, for example into a spinner flask that is slowly agitated. However, conventional static seeding is not very efficient in delivery cells into scaffolds and often results in a very low initial cell number with low uniformity within the scaffold (Li et al., Biotechnol. Prog., 2001, Vol. 17, P. 935-944). Dynamic seeding has been investigated as a more effective alternative to static seeding. For dynamic seeding the scaffold and the cell suspension are placed together in, e.g., a tube and the tube is then incubated with gentle agitation for a certain time allowing the cells to attach to the surface of the scaffold. However, the seeding efficiency for dynamic seeding was low (from 4%-56%) and variable depending on the agitation method (Byung-Soo Kim et al., Biotechnology and Bioengineering, 1998, Vol. 57 (1), P. 46-54). Various other seeding configurations which all use some kind of active force to seed the cells into a scaffold have been developed to circumvent the limitations of static and dynamic seeding. These include filtration seeding (Li et al., 2001, supra), oscillating perfusion seeding (Wendt et al., Biotechnology and Bioengineering, 2003, Vol. 84, P. 205-214), centrifuge seeding (Yang, T. H. et al., J. Biomed. Mater. Res., 2001, Vol. 55, P. 379-386) and perfusion cartridge seeding (Sittinger et al., Int. J. Artif. Organs, 1997, Vol. 20, P. 57-). However, the application of these methods are limited to scaffolds of a specific range of dimensions and pore sizes since most of the studies were performed on a single type of scaffold.
Despite the shortcomings of static seeding, this method can be employed with virtually all cell types due to its relative simplicity. Therefore, considerable efforts have been made to improve the efficiency of static seeding by using biological hydrogels such as fibrin glue (Schantz et al., Tissue Eng., 2003, Vol. 9, Suppl. 1, P. 113-126) and collagen (Ushida et al., Cell Transplant., 2002, Vol. 11(5), P. 489-494). However, the extent of gelation (complexation) of such hydrogels cannot be controlled very well owing to the inherent variability in biological materials and there is no precise control over the physiochemical parameters that triggers the gelation of these biological hydrogels. This may have implications on the diffusion limits of nutrients inside the scaffolds. Sufficient gelation can effectively trap the cells inside the scaffolds but often limit the diffusion. On the other hand, insufficient gelation will be good for mass transfer but not improving the cell seeding efficiency.
Thus, there remains a need for an effective seeding of cells in scaffolds.